Cardiac arrhythmias can be treated effectively by agents which slow conduction and/or prolong refractoriness in the heart. Class I anti-arrhythmics have their predominant effects on conduction velocity, with more modest effects on refractory period, while Class III anti-arrhythmic agents significantly prolong refractoriness without greatly altering intracardiac conduction. The electrophysiologic properties of a compound defining a Class III activity profile are observed in vivo as negligible effects on atrial, ventricular and H-V conduction time while producing a marked increase (greater than 20 percent) in both the atrial and ventricular refractory period. In contrast, Class I agents will demonstrate a marked rate-dependent slowing of ventricular conduction velocity with changes in the refractory period generally less than twenty percent (20%). Recent reviews of these agents are by Anderson, Fed. Proc. 45, 2213-2219 (1986); Bexton et al., Pharmac. Thera. 17, 315-55 (1982); Vaughan-Williams, J. Clin. Pharmacol. 24, 129-47 (1984) and Thomis et al., Ann. Rep. Med. Chem. 18, 99-108 (1983).